1 The weak base methylamine CH3NH2 reacts with water accordi

1. The weak base methylamine, CH3NH2, reacts with water according to the equation CH3NH2 (aq) + H2O (/) OH-(aq) + CH3NH3+ (aq) Kb = 4.2 x 10-4 Calculate the equilibrium hydroxide ion concentration in a 0.25 M solution of this base. What are the pH and pOH of the solution? Please show your work.

Solution

The balanced reaction with ICE TABLE

CH3NH2 + H2O <=> CH3NH3+ +OH-
Initial .. .. .. .. 0.250
React .. .. .. .. - x
Produce .. .. .. .. .. .. .. .. .. .. .. .. .. ..+ x .. .. .. . .. +x
At equil. .. .. 0.25-x .. .. .. .. .. .. .. .. x .. .. .. .. .. x
Equilibrium constant expression for the reaction

Kb= [CH3NH3+][OH-] / [CH3NH2] =x^2/(0.25-x)

4.2 x 10^-4 = x^2/(0.225-x)

-0.0000945 + 0.00042x + x² = 0

x = 0.009513

[OH-]= 0.009513 M

pOH = - log [OH-] = - log (0.009513) = 2.022

pH = 14 - pOH = 14 - 2.022 = 11.978